What are the types of validator functions?

[akhils-nair-b6a2a78b (Akhil S Nair)]

In reactive forms, the validators can be either synchronous or asynchronous functions,

1. Sync validators: These are the synchronous functions which take a control instance and immediately return either a set of validation errors or null. Also, these functions passed as second argument while instantiating the form control. The main use cases are simple checks like whether a field is empty, whether it exceeds a maximum length etc.
2. Async validators: These are the asynchronous functions which take a control instance and return a Promise or Observable that later emits a set of validation errors or null. Also, these functions passed as second argument while instantiating the form control. The main use cases are complex validations like hitting a server to check the availability of a username or email.